Make Arrow desugaring deterministic

This kills two instances of varSetElems that turned out to be
nondeterministic. I've tried to untangle this before, but it's
a bit hard with the fixDs in the middle. Fortunately I now have
a test case that proves that we need determinism here.

Test Plan: ./validate, new testcase

Reviewers: simonpj, simonmar, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2258

GHC Trac Issues: #4012